![]() ![]() The final drive ratio of this type of vehicle is 1:1 (one revolution of “transmission” equals one revolution of the wheels) so as an effect of this, these types of cars are very fast. ![]() The pinion gear on the motor drives a spur gear, which seconds as the vehicles differential and is mounted directly (hence the name direct drive) to the axle. The idler (middle) gear has no effect on the total drive reduction you can factor it into the equation, but it will cancel itself out.ĭirect drive cars are ones that have no actual transmission to speak of. This transmission’s top gear has 20 teeth, and the diferential (bottom) gear has 48 teeth for a transmission ratio of 2.4:1. That’s about as close to the desired 12:1 ratio you can get. Multiply 5.2 by the tranny ratio (2.3), and you get Just multiply it by the desired ratio:Īnd there it is: a 20T pinion and a 104T spur deliver a perfect 5.2 ratio. A nice, even number should work let’s try a 20T pinion. Instead of working back from the spur gear, try working from the pinion. If you want that elusive 100% equal ratio, you’ll have to keep looking for a pinion/spur combo that delivers a 5.2:1 ratio. You can quit now and run a slightly lower gear ratio by installing a 19T pinion, or gear slightly taller with a 20T pinion. Of course, gears don’t have fractional teeth (unless you break a tooth!). Great! If we had a 19.23T pinion, we’d be all set. We can calculate that by dividing our spur-gear tooth count by 5.2. ![]() Now we know we need a pinion-/spur-gear ratio of 5.2:1. Start by dividing the desired result (12) by 2.3: Let’s say, it’s a 2.3:1 transmission ratio. Armed with this knowledge, let’s match this setup to a car with a different ratio. That means the pinion must turn 12 times to turn the diff gear (and the rear wheels) one complete revolution. To find the total drive reduction of your transmission, spur gear and pinion combination, you must multiply the transmission ratio (2.4:1) by the pinion/spur ratio (5:1): Let’s say you’re using that 20T-pinion/100T-spur combo in the example gearbox that has a transmission ratio of 2.4:1. Once you have determined the transmission ratio, you can match your gear ratio to anyone else’s. For example, the gearbox shown below has a 20T top gear and 48T diff gear: To determine the transmission ratio, just divide the number of teeth on the diff (bottom) gear by the number of teeth on the top gear. When most people talk about their car’s gear ratio, they are referring to the number of teeth on spur (large) gear and the pinion (small) gear.Īny car with a gearbox has some type of internal gear reduction most cars and trucks use a “3-gear” transmission that features a top or “input” gear, an idler gear and a bottom gear that spins the drive axles and incorporates the differential. Even though you have bigger gears than your friend, the ratio is still the same-5:1. Which pinion should you use? Divide the spur-gear teeth by 5 (since you’re aiming for a 5:1 ratio) to find out: Let’s use the previous gearing example and say he has a 20T pinion and a 100T spur for a 5:1 ratio however, you don’t have a 100T spur of your own your only spur gear is a 125T. You have the same car, same motor and a battery of equal capacity, so you want to use his gear ratio. Let’s say your friend’s car is running well it has good acceleration and top speed, and the battery and motor aren’t being overworked. Robinson Racing offers convenient pinion “6 packs” for about $15. You don’t need to spend a lot of money to add gearing options to your pit box. The gear ratio is 5:1 (five to one), and that means that the pinion must make five revolutions to turn the spur gear one revolution. We need to go one step further, however, and divide the number of teeth on the spur gear (100) by the number of teeth on the pinion (20). Whether you’re aiming for longer run times or to set a land speed record against your friends, knowing what to change and busting out some simple math makes it easy to see how gearing works.Īny two gears will have a “ratio” if your car has a 20-tooth (20T) pinion and a 100-tooth (100T) spur gear, that’s a 20:100 ratio. Understanding your vehicle’s gear ratio is a powerful tool when making changes at the track or around the backyard. No matter what you install in your RC car or truck, when it comes to speed and run time it’s all about gearing. How to: The driving force behind your RC vehicle ![]()
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